This application claims the priority of German Application No. DE 100 41 386.2, filed in Germany on Aug. 23, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system and method for optimizing the efficiency of an oil supply comprising at least two different oil pressure levels, at least two feed devices for oil volume flow of the oil supply and at least one control slide for controlling at least one of the oil pressure levels.
In known systems for supplying oil to hydraulic systems, in particular, in such systems for automatic transmissions, a large part of the power losses are caused by the supplying of hydraulic actuators. These hydraulic actuators include, for example, clutches and variators in continuously variable transmissions (CVT). The oil supply which permits pressure to be built up at the actuators is implemented by means of one or more feed devices or oil pumps. The necessary pressure level of such pumps is between 15 and 30 bar in the case of step-by-step variable transmissions. In step-by-step variable transmissions, it is necessary to allow for up to 80 bar and higher. Owing to their high drive power, the oil pumps in automatic transmissions are therefore almost always driven by the internal combustion engine by means of a fixed transmission ratio.
The hydraulic pump power Phyd is generally as follows:
Phyd=xcex94Ppumpxc2x7Qpumpxc2x71/xcex7pump 
Here, xcex94p is the pressure difference, Q is the volume flow and xcex7 is the efficiency of the respective pump.
Because the pumps used in automatic transmissions are predominantly what are referred to as constant delivery pumps with a constant delivery volume per revolution, that is to say gearwheel or vane pumps, the delivered pump volume flow is dependent on the rotational speed of the internal combustion engine owing to the fixed transmission ratio of the drive by the internal combustion engine.
However, this disadvantage is accepted because such constant delivery pumps have advantages for high pressure levels owing to their robustness and their low cost as well as their low noise level and the possibility of implementing them in a simple technical way. In addition, these constant delivery pumps have efficiency advantages over pumps with a variable delivery volume per revolution.
In order to keep the power losses in the transmission as low as possible, the delivery volume of the oil pumps must be of such a magnitude that the oil supply is just still sufficient in all the necessary operating states of the transmission.
A problem is then that the dimensioning of the pumps must be defined with reference to operating states at a low engine speed, that is to say with a low rotational speed of the pump and resulting low delivery quantities. When the rotational speeds of the engine and of the pump are correspondingly higher, there is thus a very large excess delivery quantity, as a result of which there is an unnecessary power loss. In addition, this hydraulic power loss rises in accordance with the formula stated above as the pressure level becomes larger because the pump then has to generate a higher pressure difference xcex94p between the reservoir pressure level and the necessary pressure level.
In order to counteract this problem, it is known from the general state of the art to use an electrically switchable double chamber pump. This oil pump is divided into two chambers with different delivery volumes per revolution. One of the two chambers can be switched to a pressureless state by way of an electrical switching operation which is passed on to the pump by way of an electrohydraulic valve.
During operation, this means that both chambers feed at the same pressure level at low rotational speeds of the pump or when the actuators"" oil quantity requirement is relatively large. The double chamber pump then operates as a large constant delivery pump. As soon as relatively high rotational speeds of the pump are reached and the actuators only demand a small volume flow of oil from the oil supply, the switchable chamber is switched to a pressureless state. The double chamber pump then operates principally only as a small constant delivery pump with the delivery volume of just one of the chambers. In this case, the power loss of the double chamber pump is reduced.
However, this principle also has specific disadvantages. The step-by-step switching only partially resolves the abovementioned problem of the oil supply because within a step there continue to be large ranges in which excess oil quantities are fed.
Further disadvantages arise as a result of the electrohydraulic valve which is additionally required for switching over the pump. Said valve requires corresponding installation space and leads to additional costs. The additional valve also gives rise to further oil leakages which make it necessary to have a larger overall delivery volume of the double chamber pump and thus increase the power loss once more.
An additional disadvantage is the electrical switchability of the double chamber pump, which requires a switching strategy provided specifically for that purpose. This switching strategy is dependent on a large number of factors. These factors, which include, for example, dependences on the rotational speed of the pump, on the oil requirement of the actuators, on the change in leakage rates with temperature, on the change in leakage rates with the service life etc., make it necessary to develop a comparatively costly strategy. In addition, owing to the fluctuations and differences in the abovementioned factors this strategy cannot be an absolute strategy, but rather it must always provide reliability in terms of the delivery quantity before the operation of switching to just one of the chambers. This also disadvantageously gives rise to a further power loss.
Because the electrical switchability of the double chamber pump does not function in the case of a power failure, that is to say in emergency mode, a correspondingly high delivery quantity is always fed in this case, which in turn has disadvantageous effects on the dimensioning of the necessary cooling system so that this cooling system has to be overdimensioned for the normal operation which occurs before the predominantly greater part of the time.
In addition, in automatic transmission there is also the factor that the oil actuators, that is to say the actuators which have to be supplied by the oil pump or pumps, operate at different pressure levels. However, because the oil pump can only feed at one pressure level, the entire delivery volume flow must always be fed at the maximum required pressure of the transmission. The oil for the actuators at a lower pressure level is usually only subsequently throttled from the high pressure level to the necessary lower pressure level, resulting in further very large power losses.
An object of the invention, then, is to provide a system for supplying oil with optimized efficiency, in particular for an automatic transmission which is capable of making available at least two different oil pressure levels, and these at least two oil pressure levels are to be controlled automatically as a function of the system requirements and independently of any external auxiliary power.
This object is achieved according to certain preferred embodiments of the invention by that the at least one control slide has at least two narrowed sections, each of the narrowed sections being assigned one of the feed devices and, depending on a position of the narrowed sections in the control slide, the respective volume flows being automatically divided into the at least two pressure levels for each of the feed devices as a function of an oil requirement in the respective pressure level.
The oil supply is provided by the at least two feed devices which, according to the principle, are independent of one another in size and design. However, it would be appropriate here, in a particularly favorable embodiment of the invention, to use a double chamber pump which is uniform in its design because this constitutes a very robust, simple, space-saving and cost-effective solution.
Because the volume flow of each of the feed devices is divided to the respective pressure levels by way of a separate narrowed section of at least one control slide, it is possible that under suitable operating conditions just one of the feed devices feeds at the high pressure level while the pressure level into which the other feed device feeds has already been reduced.
The setting of the desired pressure takes place in the manner known from conventional control slides, with a narrowed section. Here, a specified pressure, which can, for example, also vary as a function of time, is applied to one effective area of the control slide. Meanwhile another, second effective area, which has a certain ratio with respect to the first effective area, has a pressure applied to it which can be set by the control slide. However, a pressure which is increased in comparison with the first specified pressure in accordance with the ratio, which is referred to as the pressure transmission ratio, of the two effective areas with respect to one another, will then be set at the outputs of the control slide. This is a hydraulic, self-regulating system which does not require any external auxiliary power.
One of the advantages of certain preferred embodiments of the system lies in the fact that, when the oil quantity requirement of the actuators supplied drops and/or the rotational speed of the feed devices increases, the pressure level of one of the pumps is reduced incrementally and the power loss associated with this pump can thus be reduced.
In addition, a further advantage is obtained by virtue of the fact that further power loss can be prevented because the two feed devices make available the necessary overall system volume, and the two pumps do not need to always feed with the maximum required pressure. The oil for the actuators which operate at the lower pressure level can, according to certain preferred embodiments of the invention, be fed only by one of the pumps in certain operating states, which pump then feeds exclusively into this lower pressure level.
A further advantage of the system according to certain preferred embodiment of the invention lies in the fact that no additional valve or the like is required to switch over the feed devices or to vary the feed pressure of the one pump because this is carried out solely by the control slides. Because control slides, which are however constructed with just one narrowed section, are, in any case, used in the equipment according to the prior art, the system, according to certain preferred embodiments of the invention, can be implemented without requiring an appreciable additional outlay in terms of costs or installation space.
In addition, the control slides comprise, as has already been mentioned above, a hydraulically self-regulating system which does not require any strategy for switching over or control. In the system according to certain preferred embodiments of the invention, the switching over of the one pump always takes place particularly advantageously at the earliest possible time. As a result, a pressure reduction occurs which is optimized in terms of efficiency and which on the one hand achieves a very high level of reliability in terms of the delivery quantity and on the other hand permits the power losses in the system to be reduced by way of the earliest possible switching over.
Even when leakage losses or the like rise owing to ageing, the system, according to certain preferred embodiments of the invention, permits automatically regulating adaptation to compensate said losses. Without a strategy, long-term monitoring or the like is necessary because the design, according to certain preferred embodiments of the invention, always automatically adjusts the necessary pressures and volume flows here in the purely hydraulic system.
The hydraulic system according to certain preferred embodiments of the invention functions here without the use of auxiliary power, which makes the system particularly suitable also for emergency operation.
According to the principle, it is, of course, possible, by calculating the delivery volumes of the two pumps, to configure the system in such a way that, for a respective specific application, for example a specific transmission, a ratio of the delivery volumes of the two feed devices with respect to one another is obtained which is optimum in terms of efficiency.
Furthermore, it is, of course, also possible to implement an oil supply with further additional feed devices, which makes even finer adaptation, and thus further reduction of the power losses, possible. Here, it is, of course, possible to make a careful selection between the expenditure in terms of the further feed devices and the benefit from the reduction of the power loss.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.